Syngeneic monoclonal antibodies (MAbs) were produced to B16 melanoma by hybridization of spleen cells from B16-F1 or B16-F10 tumor-bearing C57BL/6J mice. Two antigens were identified by the immunofluorescence reactions of anti-F1 MAbs, 2G10 (IgG1) and 3C10 (IgM). Both antigens are membrane associated in 97-99% of fixed F1 and F10 cells and are cross-reactive with normal syngeneic thymocytes, and other normal and transformed cultured mouse cells. An anti-F10 Mab, 3E9 (IgG3), reacts with a membrane antigen found in about 40% of F1 and F10 cells and in all transformed mouse cells tested, but was not found in normal cells. The 3C10 and 3E9 antigens are shed into the medium of cultured cells. 125I-Labeled membrane components of Mr 48,000 and 40,000 (3C10) and 25,000 (3E9) were immunoprecipitated. Some 3C10 immunoprecipitates also contained components of Mr 25,000 and 15,000. The three MAbs were tested for suppression of lung colonization by B16-F1 and B16-F10 metastatic variants in C57BL/6J mice. MAbs were injected 1 day prior to and 1 week after injection of tumor cells. 2G10 was highly suppressive for both B16-F1 and B16-F10 cells. 3C10 and 3E9 were suppressive for B16-F10 cells but were nonsuppressive and enhancing for B16-F1 cells (P less than or equal to 0.05). A novel immunotherapy model was tested. Delayed hypersensitivity was selectively induced to the 3C10 MAb in C57BL/6J mice by sensitization with lipid-conjugated purified MAb. Sensitized mice were injected with B16 cells and MAb to determine whether a cellular reaction with cell bound MAb in vivo could be suppressive for tumor cells. The treatment was suppressive for B16-F1 lung colonies, but caused augmentation of lung colonies from B16-F10 cells (P less than or equal to 0.05).